1. Field of the Invention
The present invention in general concerns magnetic resonance tomography (MRT) as used in medicine for the examination of patients. The present invention in particular concerns a spiral-coded method for accelerated MRT imaging as well as a magnetic resonance tomography apparatus that is suitable for implementation of the method.
2. Description of the Prior Art
MRT is based on the physical phenomenon of magnetic resonance and has been successfully used as an imaging modality for over 15 years in medicine and biophysics. In this examination modality, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal which is acquired by means of appropriate reception coils. By the use of non-homogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions. The method allows a free selection of the slice to be imaged, so slice images of the human body can be acquired in all directions. MRT as a slice image method in medical diagnostics is distinguished predominantly as a “non-invasive” examination method with a versatile contrast capability. Due to the excellent ability to representation of soft tissue, MRT developed into a method superior in many ways to x-ray computed tomography (CT). MRT today is based on the application of spin echo and gradient echo sequences that enable an excellent image quality with measurement times in the range of minutes.
The continuous technical development of the components of MRT apparatuses and the introduction of faster imaging sequences is always opening more fields of use in medicine to MRT. Real-time imaging to support minimally-invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples. In spite of the technical progress in the construction of MRT apparatuses, acquisition time of an MRT image remains the limiting factor for many applications of MRT in medical diagnostics. From a technical viewpoint (feasibility) and for reasons of patient protection (stimulation and tissue heating), a limit is set on a further increase of the capacity of MRT apparatuses with regard to the acquisition time. In recent years, multifaceted efforts have been undertaken to further reduce the image measurement time.
One approach to shorten the acquisition time is to reduce the quantity of the image data to be acquired. In order to obtain a complete image from such a reduced data set, either the missing data must be reconstructed with suitable algorithms or the deficient image must be corrected due to the reduced data.
The acquisition of the data in MRT occurs in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by means of Fourier transformation. The spatial coding of the subject that spans k-space occurs by means of gradients in all three spatial directions. Differentiation is made between the slice selection gradient (establishes an acquisition slice in the subject, typically the z-axis), the frequency coding gradient (establishes a direction in the slice, typically the x-axis) and the phase coding gradient (determines the second dimension within the slice, typically the y-axis).
Depending on the combination or interconnection of the three gradients in an imaging sequence, the scanning of k-space can ensue with a Cartesian (line-by-line) or radial or spiral trajectory (scanning path).
In the framework of the present invention, only a spiral scanning of k-space is considered. Spiral k-space trajectories were propagated for the first time in about 1981 as a possible alternative for Cartesian scanning (U.S. Pat. No. 4,307,343;). It is apparent that a spiral readout of the k-matrix with regard to a T2-weighted MRT imaging leads to an isotropic RF pulse response signal in contrast to, for example, a Cartesian scanning. For this reason the use of fast spiral scanning (fast spiral imaging)—as an equivalent to echoplanar imaging (echo planar imaging, EPI)—has increasingly gained in popularity, particularly in the fields of functional. MRT, perfusion MRT, MR spectroscopy, diffusion MRT and phase-contrast-based MRT flow measurements.
An unsolved problem in fast MRT imaging (fast single shot spiral scanning and fast multi shot spiral scanning and EPI) is that generally image quality reductions occur due to frequency and phase errors during the readout times of the RF response signal. These reductions are manifested in EPI as image distortions in the reconstructed image.
In fast spiral MRT imaging, the reconstructed image is locally cloudy and diffused, due to regionally-limited frequency displacements in k-space. In spiral. imaging, this error is generally designated as “blurring”. The causes for this are primarily susceptibility limits and inhomogeneities in the tissue of the subject to be examined. These are generally more significantly developed given higher gradient field strengths.
Such blurring can be reduced by a shortening of the readout. In the prior art this is achieved by reducing the number of the passes (repetitions) given, a constant size of the sampled region. This has the severe disadvantage, however, that missing passes result in a lower resolution of the reconstructed image. Parallel imaging techniques (PPA, partial parallel acquisition) have been considered for use to reduce the readout duration in the spiral coding. Such methods are extremely expensive in terms of calculation time and therefore are not practically applicable at the present time.